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Lecture 6 Note: {} and ∅ Are the Empty Set, a Set with No Elements

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Lecture 6 Note: {} and ∅ Are the Empty Set, a Set with No Elements Lecture 6 Note: fg and ; are the empty set, a set with no elements. Note: f;g is NOT the empty set. It is a set with one element, the element that is the empty set. Example: In set notation, write the set of positive integers less than 1000 and which are multiples of 7. Instructor's Comments: Might be good to give students a minute to try this Solution: fn 2 N : n < 1000 ^ 7 j ng. Another answer is given by f7k : k 2 N ^ k ≤ 142g Note: The : symbol means \such that". Sometimes j is used as well (though because we use it for divisibility, we won't use it in this context very often if at all). Instructor's Comments: This is the 7 minute mark 1 Handout or Document Camera or Class Exercise Describe the following sets using set-builder notation: (i) Set of even numbers between 5 and 14 (inclusive). (ii) All odd perfect squares. (iii) Sets of three integers which are the side lengths of a (non-trivial) triangle. (iv) All points on a circle of radius 8 centred at the origin. Instructor's Comments: 5 minutes to try on their own and 5 to take up Solution: (i) f6; 8; 10; 12; 14g or fn 2 N : 5 ≤ n ≤ 14 ^ 2 j ng (ii) f(2k + 1)2 : k 2 Zg (or N overlap doesn't matter!) (iii) f(a; b; c): a; b; c 2 N ^ a < b + c ^ b < a + c ^ c < a + bg (iv) f(x; y): x; y 2 R ^ x2 + y2 = 82g Instructor's Comments: This is the 17 minute mark 2 Set Operations. Let S and T be sets. Define (i)# S or jSj. Size of the set S. (ii) S [ T = fx : x 2 S _ x 2 T g (Union) (iii) S \ T = fx : x 2 S ^ x 2 T g (Intersection) (iv) S − T = fx 2 S : x2 = T g (Set difference) (v) S¯ or Sc (with respect to universe U) the complement of S, that is Sc = fx 2 U : x2 = Sg = U − S (vi) S × T = f(x; y): x 2 S ^ y 2 T g (Cartesian Product) Example: (1; 2) 2 Z × Z, (2; 1) 2 Z × Z, BUT (1; 2) 6= (2; 1). Note: Z × Z and f(n; n): n 2 Zg are different sets!!! Example: Z = fm 2 Z : 2 j mg [ f2k + 1 : k 2 Zg ; = fm 2 Z : 2 j mg \ f2k + 1 : k 2 Zg Instructor's Comments: This is the 30-33 minute mark Definition: Let S and T be sets. Then (i) S ⊆ T : S is a subset of T . Every element of S is an element of T . (ii) S ( T : S is a proper/strict subset of T . Every element of S is an element of T and some element of T is not in S. (iii) S ⊇ T : S contains T . Every element of T is an element of S. (iv) S ) T : S properly/strictly contains T . Every element of T is an element of S and some element of S is not in T . Definition: S = T means S ⊆ T and T ⊆ S. Example: f1; 2g = f2; 1g Example: Prove fn 2 N : 4 j (n + 1)g ⊆ f2k + 1 : k 2 Zg Proof: Let m 2 fn 2 N : 4 j (n + 1)g. Then 4 j (m + 1). Thus, 9` 2 Z such that 4` = m + 1. Now m = 2(2`) − 1 = 2(2`) − 2 + 2 − 1 = 2(2` − 1) + 1: Hence m 2 f2k + 1 : k 2 Zg. Instructor's Comments: This is the 40-43 minute mark. You might run out of time in the next example. Carry forward to Lecture 7 as need be. Example: Show S = T if and only if S \ T = S [ T . 3 Proof: Suppose S = T . To show S \ T = S [ T we need to show that S \ T ⊆ S [ T and that S \ T ⊇ S [ T First suppose that x 2 S \ T . Then x 2 S and x 2 T . Hence x 2 S [ T . Next, suppose that x 2 S [ T . Then x 2 S or x 2 T . Since S = T we have in either case that x 2 S and x 2 T . Thus x 2 S \T . This shows that S \T = S [T and completes the forward direction. Now assume that S \ T = S [ T . We want to show that S = T which we do by showing that S ⊆ T and T ⊆ S. First, suppose that x 2 S. Then x 2 S [ T = S \ T . Hence x 2 T . Next, suppose that x 2 T . Then x 2 S [ T = S \ T . Hence x 2 S. Therefore, S = T . Instructor's Comments: The last two points give a good learning moment to explain when the word `similarly' can be used. This is the 50 minute mark. 4.
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